1. Field of the Invention
The present invention relates to a map displaying apparatus, and more particularly to a three-dimensional map displaying apparatus which displays routes on a three-dimensional map.
2. Description of the Background Art
There has been a class of map displaying apparatuses (such as navigation apparatuses) which allow two-dimensional (2D) map data to be viewed from different viewpoints, or which display a map in a three-dimensional manner based on three-dimensional (3D) map data which is previously constructed from polygons. Hereinafter, such apparatuses will be referred to as “3D map displaying apparatuses”. A 3D map displaying apparatus permits a user to have an intuitive understanding of a map. FIG. 21 is a diagram illustrating an exemplary image displayed by a conventional 3D map displaying apparatus. As seen from FIG. 21, a 3D map displaying apparatus can provide a realistic image representation.
Some 3D map displaying apparatuses have the function of searching for an optimum route based on a start point and a destination point which have been set by a user and displaying a found route so as to be overlaid on a 3D map, and/or the function of receiving neighborhood traffic information by the use of communication means and displaying congested sites and closed sites, if any, so as to be overlaid on a 3D map. Displaying a three-dimensional map on an apparatus having such functions is especially useful because it permits a user to have an intuitive understanding of the routes and traffic information. For example, there are techniques for displaying a 3D map such that guide lines indicating routes or road information are displayed with a perspective “depth”, in an effort to provide for a better visual recognition.
According to the aforementioned techniques, every route that is displayed on the screen is displayed so as to appear upright (i.e., appear to have a “height” dimension”). In other words, not only routes extending along the right-left direction on the on the screen but also routes extending along the “depth” direction on the screen are displayed so as to appear upright, although the latter routes are generally not difficult to recognize and therefore do need not to be displayed so as to appear upright. What is worse, routes extending along the depth direction may actually become more difficult to recognize when appearing upright. Moreover, the aforementioned techniques are directed to providing upright displayed images of routes and road information, and therefore cannot improve the visual recognition of any other displayed objects, e.g., roads, train stations, and the like.
Furthermore, according to the above-described three-dimensional displaying techniques, any length along the depth direction of the screen will appear disproportionately short. This makes it very difficult to recognize any displayed objects (e.g., roads or routes) extending along the right-left direction of the display screen, which in turn makes it difficult for the user to properly understand the displayed information. For example, in the example shown in FIG. 21, roads extending along the right-left direction of the display screen (i.e., roads 71 to 74) appear so narrow that they may be difficult to recognize. Therefore, the user may overlook any number of the roads to take right or left at from an intersection, or the user may lose track of the roads on a displayed route. Similarly, a train station 76 extending along the right-left direction of the display screen appears so narrow that it may be difficult to recognize. Thus, according to the conventional 3D map displaying techniques, the objects displayed as 3D images may become difficult to visually recognize, such that the user cannot properly understand the displayed map.
A method for solving the above problem might be to set broad widths in the map data, or elevate the viewpoint on the screen. However, setting broad road widths in the map data itself would cause, not only the roads extending along the right-left direction of the screen, but also those extending along the depth direction to appear broad. For example, as shown in FIG. 21, a road 75 extending along the “depth” direction also appears broad, although such a road 75 is relatively easy to recognize without being provided with such a broad width. If any other displayed objects are present on the sides of a road extending along the depth direction, a broadened road width will be in an overlapping relationship with the other displayed objects, whereby the other displayed objects may become all the more difficult to recognize. On the other hand, elevating the view point on the screen is in itself a limitation on the viewpoint, which makes it difficult to represent a realistic map that is taken from a viewpoint similar to the user's viewpoint. Thus, there has been no effective conventional means for improving the recognition of those displayed objects whose recognizability worsens when displayed as 3D images.